1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, particularly to a method of manufacturing a DRAM (Dynamic Random Access Memory) in which the short margin of a fine capacitance contact formed by self-alignment is increased.
2. Description of the Prior Art
In recent DRAMs, the adoption of the COB (Capacitor Over Bitline) construction has become mainstream because it is easy to ensure the capacity of a capacitor. In a memory cell of the COB construction, a word line which is formed on the surface of a semiconductor substrate, a bit line which is arranged on the word line via an interlayer insulating film so as to be orthogonal to the word line, and a capacitor which is formed above the bit line via an interlayer insulating film are main component elements. In order to ensure that short circuits are not formed with the word line and the bit line, the capacitor which is present in the highest position is connected to the semiconductor substrate via a contact plug which is formed by threading the gaps between the lines.
Hereinafter, a memory cell of the above-described COB construction will be further described by using the sectional view shown in FIG. 1. FIG. 1 is shown in a section in a direction parallel to a word line (perpendicular to a bit line).
An element isolation region 102, and a drain 103 and a source 104 which consist of an n-type diffusion layer are provided in a prescribed region of the surface of a p-type semiconductor substrate 101. A first interconnect layer 105, which becomes a word line, is provided via a gate insulating film formed on the surface of the semiconductor substrate 101, and the first interconnect layer 105 is covered with a first interlayer insulating film 106. First contact plugs 107 and 108 are provided in a prescribed region of the first interlayer insulating film 106. A second interlayer insulating film 109 is provided on the surfaces of the first contact plugs 107, 108 and the first interlayer insulating film, and a second contact plug 110, which becomes a bit line contact plug, is provided so as to establish a connection to the first contact plug 107. A second interconnect layer 111, which becomes a bit line, is provided on the second contact plug 110, and the second interconnect layer is covered with a third interlayer insulating film 112. A third contact plug 113, which becomes a capacitance contact plug, is provided in the third interlayer insulating film 112 so as to establish a connection to the first contact plug 108 between the second interconnect layers 111. A fourth interlayer insulating film 114 is provided on the surfaces of the third contact plug 113 and the third interlayer insulating film, a cylinder hole is provided at a position corresponding to the third contact plug in the fourth interlayer insulating layer, and a lower electrode 115 of the capacitor is provided on an inner surface of the cylinder hole so as to establish a connection to the third contact plug. A capacitance insulating film 116 and an upper electrode 117 are provided so as to cover the lower electrode 115. Furthermore, a third interconnect layer 119 is provided via a fifth interlayer insulating film 118, whereby a memory cell of the COB construction is formed.
In a DRAM of the above-described COB construction, because of requirements for an improvement in integration level, a memory cell is keeping on decreasing in size. For this reason, also the plane area allowed for each component element must be decreased and the formation of each of the above-described contact plugs has also become very difficult. In particular, in the formation of the capacitance contact plug (the third contact plug) formed between adjacent bit lines (the second interconnect layer), it is necessary to increase the thickness of the interlayer insulating film which insulates the bit line and the capacitor in order to ensure the fabrication margin of the capacitor, and for this reason, the working margin becomes small, thereby bringing the formation of the capacitance contact plug into a more difficult situation. The SAC (Self Aligned Contact) method is adopted in order to reduce this difficulty.
Hereinafter, a method of manufacturing a capacitance contact plug by a conventional SAC method will be described in detail by using sectional views of a series of steps shown in FIGS. 2A to 2E. In these figures, a semiconductor substrate and a word line formed on the semiconductor substrate are omitted.
First, as shown in FIG. 2A, first contact plugs 107 and 108 are formed in a prescribed region of a first interlayer insulating film 106 which covers a word line. Next, a second interlayer insulating layer 109 which consists of a silicon oxide film having a thickness of 150 nm is formed, and a second contact plug 110 is formed so as to establish a connection to the first contact plug 107. After that, a film of a metal material having a thickness of 70 nm is formed, which becomes a bit line, and on top of this film a silicon nitride film 120 having a thickness of 60 nm is further formed. The silicon nitride film 120 and the film of the metal material are worked by lithography and dry etching, whereby a bit line 111 is formed. After that, a side wall which consists of a silicon nitride film 121 having a thickness of 20 nm is formed by a publicly known method.
Next, as shown in FIG. 2B, a third interlayer insulating film 112 which consists of a silicon oxide film having a thickness of 800 nm is formed overall, the surface is planarized by the CMP (Chemical Mechanical Polishing) method so that the remaining film thickness provides the third interlayer insulating film 112 of 400 nm. A silicon film 122 having a thickness of 80 nm is formed on top of the third interlayer insulating film 112. Furthermore, a photoresist 123 is formed and a prescribed pattern is formed by a publicly known method.
Next, as shown in FIG. 2C, the silicon film 122 is dry etched by using the photoresist 123 as a mask and the pattern is transferred. The silicon film 122 on which the pattern has been transferred is used as a hard mask to dry etch a lower-layer insulating film. After that, the third interlayer insulating film 112 which consists of the silicon oxide film having a thickness of 400 nm and the second interlayer insulating film 109 which consists of the silicon oxide film having a thickness of 150 nm are etched and a contact hole 124 is formed. At this time, the silicon nitride films 120 and 121 which cover the bit line 111 have a lower etching rate than the silicon oxide film, and hence can form the contact hole of the silicon oxide film in a self-aligning manner and the bit line 111 will not be exposed even when the end portion of the contact hole 124 is present above the bit line 111.
Next, as shown in FIG. 2D, a polycrystalline silicon film 125 which contains phosphorus is formed in such a manner that the contact hole 124 is buried. Subsequently, as shown in FIG. 2E, the polycrystalline silicon 125 on the surface is removed by the CMP method, and a third contact plug 113 made of polycrystalline silicon is formed.
Methods of forming a contact plug which are similar to the above-described method are disclosed in Japanese Patent Application Laid-Open No. 2001-102550 and Japanese Patent Application Laid-Open No. 2004-304141.
However, with progress in miniaturization, it has become difficult to form high-reliability contact holes even by use of the above-described SAC method. In the SAC method, by covering a bit line with a silicon nitride film having a lower etching rate than a silicon oxide film, it is ensured that the bit line is not exposed during the etching of the silicon oxide film. The etching rate ratio between silicon oxide and silicon nitride in dry etching is 5 to 7 or so, and it is difficult to dramatically change this value even by changing dry etching conditions. This is because both silicon oxide and silicon nitride are silicon compounds and in an environment of dry etching it is difficult to expand the difference in the etching rate. Hereinafter, an investigation will be made into a thickness of a silicon nitride film which remains on a bit line in the part indicated by a circle mark A in FIG. 2C under this condition when the above-described related art is used.
The thickness of a silicon oxide film which must be etched after the exposure of the surface of a silicon nitride film formed on a bit line is 280 nm in total, of which 60 nm are for the thickness of the silicon nitride film, 70 nm are for the thickness of the bit line and 150 nm are for the thickness of the second interlayer insulating film. If the etching rate ratio between the silicon oxide film and the silicon nitride film is 7, the silicon nitride film is etched by about 40 nm during the etching of the silicon oxide film by 280 nm. Because the thickness of the silicon nitride film formed on the bit line is 60 nm, a silicon nitride film having a thickness of 20 nm remains. If the silicon nitride film having a thickness of 20 nm remains, no short circuit occurs between the third contact plug 113 and the bit line 111 in the part indicated by a circle mark A in FIG. 2E.
However, when the contact hole diameter decreases, the deeper a contact hole, the lower the etching rate will be and it becomes impossible to maintain the above-described etching rate ratio. That is, in the etching of the second interlayer insulating film shown in FIG. 2C, the etching rate becomes lower and the etching rate ratio with respect to the silicon nitride film decreases to about 4. As the result, the silicon nitride film having a thickness of 60 nm formed on the bit line is completely etched before the surface of the first contact plug 108 is exposed, thereby posing the problem that the third contact plug 113 and the bit line 111 form a short circuit in the part indicated by a circle mark A. Although this problem can be solved to a certain extent by increasing the thickness of the silicon nitride film, another problem will arise; for example, the formation of the third interlayer insulating film 112 will undesirably become difficult.
In view of the above problem, the object of the present invention is to provide a method of forming a high-reliability contact plug which prevents a short circuit between the contact plug and a bit line by applying a material having a large etching rate so that the etching rate ratio with respect to a silicon nitride film in an interlayer film which forms the contact plug is infinite, to prevent the etching of the silicon nitride film during the etching of the interlayer film, with the result that the thickness of the silicon nitride film remaining on the bit line is ensured. The object of the present invention is also to provide a method of manufacturing a semiconductor device in which this method of forming the contact plug is used.